Semiconductor laser devices such as ridge waveguide lasers are used in many fiber optic communications systems. Incremental refinements in their fabrication and packaging have resulted in a class of devices that have acceptable performance characteristics and a well-understood long-term behavior. Moreover, the ridge waveguide structures are less complex to fabricate and provide excellent yields as compared to more complex architectures based on buried heterostructures, for example.
Single wavelength lasers have been essential in moderate and long distance ( greater than 10 km) fiber optic communication systems. Multimode or multi-wavelength optical transmitters suffer restrictions on the distance information can be sent since pulse spreading is proportional to the product of differential group delay of the fiber and optical bandwidth of the source. For this reason, lasers with multiple transverse modes, multiple lateral modes, or multiple longitudinal modes are to be avoided for moderate and long distance optical communications.
Distributed feedback (DFB) lasers are typically used as optical sources having robust singlemode output. However, direct modulation of a DFB laser tends to induce rapid modulation of the laser index and extensive shifting (or chirp) in the laser oscillation wavelength. Especially large changes in the properties of the laser material are required to rapidly extinguish and rebuild the photon density within the laser cavity on time scales of less than 100 ps. Direct modulated DFBs typically generate a factor of two excess in optical bandwidth. Thus, direct modulated DFBs have inadequate spectra for large signal modulation data rates of 10 Gbit/s and beyond.
Generally, several solutions have been adopted to avoid the chirp of direct modulation. External modulators (using such materials as LiNO3) have produced excellent chirp behavior. However, due to the dissimilarity of the laser and modulator structures, the complexity and thus the size and cost of the resulting transmitter is significantly increased when external modulation is used. Integrated laser and semiconductor Mach-Zehnder modulators have been demonstrated. However, results so far have shown that these devices have high insertion loss.
Semiconductor waveguide modulators have been integrated with the laser chip to permit subsequent modulation after the laser cavity, eliminating the need to quench and regenerate the photon density within the laser cavity. These have been demonstrated in devices known as electroabsorption modulated lasers (EMLs).
There remains a continuing need for improvements to increase output power, efficiency and spectral characteristics for integrated laser modulator devices.
It would be useful to maintain the advantages of near optimum chirp behavior without trading off a significant fraction of DFB power (permitting one to get  greater than 5 mW from the modulator). It would also be useful to maintain the advantage of simple, cheap integration of source and modulator into a single chip.
Electroabsorption modulated laser devices in accordance with the present invention provide increased output power, higher quantum efficiency, and better spectral characteristics compared to typical integrated laser modulators.
According to one aspect of the present invention, an integrated electroabsorption modulated laser (EML) device includes a distributed feedback (DFB) laser and modulator. The DFB laser includes an active layer and a complex index grating. The modulator includes an active layer. The EML device includes a first electrical contact over the DFB laser and a second electrical contact over the modulator. The EML device includes a stop etch layer above the active layer of both the DFB laser and the modulator.
In another aspect, an integrated electroabsorption modulated laser (EML) device includes a distributed feedback (DFB) laser, an amplifier and a modulator and is referred to herein as an electroabsorption modulated partial grating laser (EMPGL) device.
In an embodiment of an EMPGL device, the DFB laser includes an active layer and a complex index grating. The amplifier includes an active layer and a grating-free region. The modulator includes an active layer. The EMPGL device includes a common electrical contact over the DFB laser and amplifier and has a separate electrical contact over the modulator. An ion implantation region in the EMPGL device provides electrical isolation between the DFB laser/amplifier and the modulator.
In another embodiment of an EMPGL device, the EMPGL device includes a modulator between a DFB laser and an amplifier. A first ion implantation region provides electrical isolation between the DFB laser and the modulator and a second ion implantation provides electrical isolation between the amplifier and the modulator. The EMPGL device has separate electrical contacts over the DFB laser, amplifier and modulator.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.